Lateritic ore deposits contain most of the Ni and Co resources in the earth's crust. The known reserve of Ni in laterite ores is about three times that of the sulfide ores. However, until recently the majority of Ni and Co productions came from sulfide ore deposits, which are expensive to mine, but can be readily concentrated. Ni and Co can be extracted from lateritic ores using a pyrometallurgical process involving drying, calcining, and smelting; however, this technology is best suited to the serpentinic fraction of the ore. It is unsuitable for high-iron laterite ores called limonitic ores, because of low-grade ferronickel product and high production cost.
Recent improvements in the technology of pressure acid leaching make the hydrometallurgical process an attractive route for processing limonitic ore deposits. Such process is disclosed, for example, by D. Georgiou and V. G. Papangelakis in an article entitled “Sulphuric acid pressure leaching of a limonitic laterite: chemistry and kinetics” Hydrometallurgy 49 (1998) 23-46. Also, E. T. Carlson and C. S. Simons provide a good description of such process in “Pressure Leaching of Nickeliferous Laterites with Sulfuric Acid” published in Extractive Metallurgy of Copper, Nickel and Cobalt, edited by P. Queneau, Interscience Publishers, New York, N.Y. (1961) pp. 363-397. Another description of such process is provided by W. P. C. Duyvesteyn, G. R. Wicker and R. E. Doane in an article entitled “An Omnivorous Process for Laterite Deposits” given at International Laterite Symposium, New Orleans, La. (1979).
The feed to such hydrometallurgical process is a lateritic ore slurry with a pulp density of 25 to 50% solids. The slurry is heated in an autoclave to a temperature in the range 240 to 270° C., which corresponds to a pressure range of 500-800 psi, then contacted with sulfuric acid. The acid is added in sufficient amounts to give a residual free acid concentration of 30 to 50 g/L at room temperature before flashing. The retention time ranges from 25 to 105 minutes. Under these conditions most of the contained Ni and Co enter the solution; the extraction levels are typically 95% and 91% for Ni and Co, respectively.
During such high pressure acid leaching, impurity elements also enter the solution and can be detrimental to downstream processes. Al and Fe hydrolyze and precipitate leaving only small amounts thereof in the leach liquor. The residual Al and Fe concentrations are subsequently precipitated in a partial neutralization step. Cr is another important impurity in the leach liquor. About 4-25% of the Cr in the feed is leached, and under the oxidizing conditions of the autoclave, the dissolved Cr is in the hexavalent state, Cr2O7−2. The concentration of Cr(VI) in the liquor depends on the feed composition. A typical Cr(VI) concentration ranges from 300 to 1500 ppm. Hexavalent chromium is an environmental hazard and impacts downstream processes. For example, Cr poisons the organics in solvent extraction and makes solvent extraction an expensive option. Also, Cr(VI) is not readily precipitated and must be converted to the trivalent state for subsequent easier removal downstream. A method for the rejection of Cr is therefore highly desirable.